1. Field of the Invention
This invention relates to an apparatus for launching angularly separated mode groups in a light guide, such as an optical fiber.
2. Description of Related Art
Light guides, such as optical fibers and gradient index rods, can transmit angularly separated mode groups of optical energy. Angular separation of the mode groups may be achieved by introducing the optical energy to a receiving fiber from separate light sources at different entry angles, i.e., at different angles with respect to the longitudinal axis of the receiving fiber. The angularly separated mode groups can be simultaneously transmitted through an optical fiber.
This modal or angle multiplexing may be used, for example, to transmit information or in an intrusion detection system. Intrusion detection information can be obtained by appropriately comparing the modes transmitted by the optical fiber.
To select and launch these mode groups into a receiving fiber in a controlled manner is very difficult. One reason for this is that the receiving fiber typically has a very small diameter as compared to the relevant dimensions of the light source that must be used. Consequently, there may be a substantial power loss in launching of the optical energy into the fiber. In addition, a typical light source, such as a light emitting diode (LED) often emits light over a very broad range of angles. This broad range of angles reduces the modal purity, and the resulting combinations of power and modal purity that can be achieved are reduced. Modal purity is measured by the extent to which mode groups mix together and interfere with each other with a higher modal purity being obtained when such mixing and interfering are reduced.
In an attempt to address these problems, efforts have been made to very precisely control the alignment and tolerance accumulations inherent in manufacture and assembly. Unfortunately, this approach is costly, makes manufacture difficult and is sensitive to environmental conditions.